Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semiconductive material disposed there between in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semiconductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semiconductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semiconductive material to accept, conduct and donate holes or electrons can be enhanced by doping the material.
For example, a p-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO level of the semiconductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO level of the semiconductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form a thin film transistor (TFT). When an organic material is used as the semiconductive material in such a device, it is known as an organic thin film transistor (OTFT).
OTFTs may be manufactured by low cost, low temperature methods such as solution processing. Moreover, OTFTs are compatible with flexible plastic substrates, offering the prospect of large-scale manufacture of OTFTs on flexible substrates in a roll-to-roll process.
With reference to FIG. 4, the general architecture of a bottom-gate organic thin film transistor (OTFT) comprises a gate electrode 12 deposited on a substrate 10. An insulating layer 11 of dielectric material is deposited over the gate electrode 12 and source and drain electrodes 13, 14 are deposited over the insulating layer 11 of dielectric material. The source and drain electrodes 13, 14 are spaced apart to define a channel region therebetween located over the gate electrode 12. An organic semiconductor (OSC) material 15 is deposited in the channel region for connecting the source and drain electrodes 13, 14. The OSC material 15 may extend at least partially over the source and drain electrodes 13, 14.
Alternatively, it is known to provide a gate electrode at the top of an organic thin film transistor to form a so-called top-gate organic thin film transistor. In such an architecture, source and drain electrodes are deposited on a substrate and spaced apart to define a channel region there between. A layer of an organic semiconductor material is deposited in the channel region to connect the source and drain electrodes and may extend at least partially over the source and drain electrodes. An insulating layer of dielectric material is deposited over the organic semiconductor material and may also extend at least partially over the source and drain electrodes. A gate electrode is deposited over the insulating layer and located over the channel region.
An organic thin film transistor can be fabricated on a rigid or flexible substrate. Rigid substrates may be selected from glass or silicon and flexible substrates may comprise thin glass or plastics such as poly(ethylene-terephthalate) (PET), poly(ethylene-naphthalate) (PEN), polycarbonate and polyimide.
The organic semiconductive material may be made solution processable through the use of a suitable solvent. Exemplary solvents include mono- or poly-alkylbenzenes such as toluene and xylene; tetralin; and chloroform. Preferred solution deposition techniques include spin coating and ink jet printing. Other solution deposition techniques include dip-coating, roll printing and screen printing.
The length of the channel defined between the source and drain electrodes may be up to 500 microns, but preferably the length is less than 200 microns, more preferably less than 100 microns, most preferably less than 20 microns.
The gate electrode can be selected from a wide range of conducting materials for example a metal (e.g. gold) or metal compound (e.g. indium tin oxide). Alternatively, conductive polymers may be deposited as the gate electrode. Such conductive polymers may be deposited from solution using, for example, spin coating or ink jet printing techniques and other solution deposition techniques discussed above.
The insulating layer comprises a dielectric material selected from insulating materials having a high resistivity. The dielectric constant, k, of the dielectric is typically around 2-3 although materials with a high value of k are desirable because the capacitance that is achievable for an OTFT is directly proportional to k, and the drain current ID is directly proportional to the capacitance. Thus, in order to achieve high drain currents with low operational voltages, OTFTs with thin dielectric layers in the channel region are preferred.
The dielectric material may be organic or inorganic. Preferred inorganic materials include SiO2, SiNx and spin-on-glass (SOG). Preferred organic materials are generally polymers and include insulating polymers such as poly vinylalcohol (PVA), polyvinylpyrrolidine (PVP), acrylates such as polymethylmethacrylate (PMMA), fluorinated polymers and benzocyclobutanes (BCBs) available from Dow Corning. The insulating layer may be formed from a blend of materials or comprise a multi-layered structure.
The dielectric material may be deposited by thermal evaporation, vacuum processing or lamination techniques as are known in the art. Alternatively, the dielectric material may be deposited from solution using, for example, spin coating or ink jet printing techniques and other solution deposition techniques discussed above.
If the dielectric material is deposited from solution onto the organic semiconductor, it should not result in dissolution of the organic semiconductor. Likewise, the dielectric material should not be dissolved if the organic semiconductor is deposited onto it from solution. Techniques to avoid such dissolution include: use of orthogonal solvents for example use of a solvent for deposition of the uppermost layer that does not dissolve the underlying layer; and cross linking of the underlying layer.
The thickness of the insulating layer is preferably less than 2 micrometers, more preferably less than 500 nm.
Organic semiconductors are a class of organic molecules having extensively conjugated pi systems allowing for the movement of electrons.
Preferred methods for preparation of these molecules are Suzuki reactions (coupling or polymerization reactions) as described in, for example, WO 2000/53656 and Yamamoto polymerization as described in, for example, T. Yamamoto, “Electrically Conducting And Thermally Stable pi-Conjugated Poly(arylene)s Prepared by Organometallic Processes”, Progress in Polymer Science 1993, 17, 1153-1205. These techniques both operate via a “metal insertion” wherein the metal atom of a metal complex catalyst is inserted between an aryl group and a leaving group of a monomer. In the case of Yamamoto polymerization, a nickel complex catalyst is used; in the case of Suzuki reaction, a palladium complex catalyst is used.
For example, in the synthesis of a linear polymer by Yamamoto polymerization, a monomer having two reactive halogen groups is used. Similarly, according to the method of Suzuki reaction, at least one reactive group is a boron derivative group such as a boronic acid or boronic ester and the other reactive group is a halogen. Preferred halogens are chlorine, bromine and iodine, most preferably bromine.
Alternatively, stannyl groups may be used as reactive groups in polymerization or coupling reactions (Stille reactions).
The performance of organic semiconductors is typically assessed by measurement of its “charge mobility” (cm2 V−1 s−1), which may relate to either the mobility of holes or electrons. This measurement relates to the drift velocity of charge carriers to an applied electric field across a material.
Organic semiconductors having relatively high mobilities tend to be those which comprise compounds having a rigid planar structure with extensive conjugation which allows for efficient and effective pi-pi stacking in the solid state.
WO 2007/068618 describes a variety of organic semiconductors, each comprising an array of fused aromatic rings having a central benzene ring substituted with acetylene groups.
JP 2007/088222 and WO 2007/116660 describe the use of benzodithiophenes and its derivatives in small molecule, oligomeric and polymeric form, as organic semiconductors.
However, the increased level of conjugation required to allow compounds to form such a pi-pi stack may also result in a decrease in band gap and stability of the semiconductor, leading to poor performance and a short lifetime. Moreover, these compounds may be highly insoluble due to the size of molecule required to achieve extended conjugation, which poses particular problems in synthesis and renders their use in efficient transistor production methods, such as ink-jet printing, difficult.